Effect of nano-ZnO with different particle size on the performance of PVDF composite membrane

To evaluate the effect of nanoparticle size on the structure and performance of the membranes, polyvinylidene fluoride composite membranes modified by nano-ZnO with different particle size were successfully fabricated by the non-solvent induced phase inversion method. A series of tests, including contact angle measurement, porosity and pore size measurement, scanning electron microscope, atomic force microscopy, differential scanning calorimeter and filtration experiments were performed to characterise the composite membranes. The results showed that the size of nano-ZnO particle has an influence on the structure and performance of the composite membranes. The membrane with 90?nm nano-ZnO particle has abundant porosity, high hydrophilicity and low surface roughness. These features are responsible for its excellent permeability and antifouling property.     

Electrodepositing Pt on a Nafion-bonded carbon electrode as a catalyzed electrode for oxygen reduction reaction

The research aims to increase the utilization of platinum (Pt) catalysts and thus to lower the catalyst loadings in the electrode for oxygen reduction reaction (ORR). The electrodeposition of Pt was performed on a rotation disk electrode (RDE) of glass carbon (GC), on which a layer of Nafion-bonded carbon of Vulcan XC 72R was dispersed in advance. The behaviors of Pt RDE and GC RDE in an aqueous solution containing HCl and H₂PtCl₆ were firstly studied. It was found that Pt deposition could be achieved if the electrode potential is controlled below −0.20 V versus (saturated-potassium-chloride silver chloride electrode) SSCE. However, quite a high overpotential is necessary if a quick and apparent deposition were required. Unfortunately, at a high overpotential, the hydrogen evolution would be unavoidable and even accelerated by the formation of nanometer size of Pt particles on the RDE. It was found that it is futile to increase platinum deposits just through extending the deposition time. It was also found that too large deposition current is not helpful for increase of platinum deposition because most of the current was consumed on hydrogen evolution in this case. It has been confirmed that it is conducive to richen Pt ions, present in the form of anionic complex in solution, onto the working electrode to be deposited. It is also helpful to eliminate the hydrogen bubbles formed on the working electrode, i.e., uncatalyzed carbon electrode (UCE), by imposing a positive current on the UCE for a length of time in advance of each cathodic deposition. The potential changes during deposition were recorded. Cyclic voltammograms (CV) of electrodes in 0.5 M H₂SO₄ before and after the deposition were used to assess loading of metal catalysts in a wide range of potential from −0.20 to 1.1 V versus SSCE. The results have shown that the performance of such an electrode with loadings estimated to be 50 μg Pt/cm² is much better than those of a conventional electrode with loadings of 100 μg Pt/cm².     

Influence of carbon support on the performance of platinum based oxygen reduction catalysts in a polymer electrolyte fuel cell

Novel carbons from the Sibunit family prepared via pyrolysis of hydrocarbons [Yermakov YI, Surovikin VF, Plaksin GV, Semikolenov VA, Likholobov VA, Chuvilin AL, Bogdanov SV (1987) React Kinet Catal Lett 33:435] possess a number of attractive properties for fuel cell applications. In this work Sibunit carbons with BET surface areas ranging from ca. 20 to 420 m² g⁻¹ were used as supports for platinum and the obtained catalysts were tested as cathodes in a polymer electrolyte fuel cell. The metal loading per unit surface area of carbon support was kept constant in order to maintain similar metal dispersions (∼0.3). Full cell tests revealed a strong influence of the carbon support texture on cell performance. The highest mass specific activities at 0.85 V were achieved for the 40 and 30 wt.% Pt catalysts prepared on the basis of Sibunit carbons with BET surface areas of 415 and 292 m² g⁻¹. These exceeded the mass specific activities of conventional 20 wt.% Pt/Vulcan XC-72 catalyst by a factor of ca. 4 in oxygen and 6 in air feed. Analysis of the I–U curves revealed that the improved cell performance was related to the improved mass transport in the cathode layers. The mass transport overvoltages were found to depend strongly on the specific surface area and the texture of the support.     

Kinetics and PEMFC performance of RuxMoySez nanoparticles as a cathode catalyst

Kinetics of Ru_xMo_ySe_z nanoparticles dispersed on carbon powder was studied in 0.5 M H₂SO₄ electrolyte towards the oxygen reduction reaction (ORR) and as cathode catalysts for a proton exchange membrane fuel cell (PEMFC). Ru_xMo_ySe_z catalyst was synthesized by decarbonylation of transition-metal carbonyl compounds for 3 h in organic solvent. The powder was characterized by X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. Catalyst is composed of uniform agglomerates of nanocrystalline particles with an estimated composition of Ru₆Mo₁Se₃, embedded in an amorphous phase. The electrochemical activity was studied by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. Tafel slopes for the ORR remain invariant with temperature at −0.116 V dec⁻¹ with an increase of the charge transfer coefficient in dα/dT = 1.6 × 10⁻³, attributed to an entropy turnover contribution to the electrocatalytic reaction. The effect of temperature on the ORR kinetics was analyzed resulting in an apparent activation energy of 45.6 ± 0.5 kJ mol⁻¹. The catalyst generates less than 2.5% hydrogen peroxide during oxygen reduction. The Ru_xMo_ySe_z nanoparticles dispersed on a carbon powder were tested as cathode electrocatalyst in a single fuel cell. The membrane-electrode assembly (MEA), included Nafion^® 112 as polymer electrolyte membrane and commercial carbon supported Pt (10 wt%Pt/C-Etek) as anode catalyst. It was found that the maximum performance achieved for the electro-reduction of oxygen was with a loading of 1.0 mg cm⁻² Ru_xMo_ySe_z 20 wt%/C, arriving to a power density of 240 mW cm⁻² at 0.3 V and 80 °C.     

Preparation of carbon-supported nano-sized LaMnO3 using reverse micelle method for energy-saving oxygen reduction cathode

Two ways of reverse micelle (RM) method were investigated to prepare carbon-supported nano-sized LaMnO₃ with high oxygen reduction activity. Hydrolysis precipitation in reverse micelle (HP-RM) method could give nano-sized particles of LaMnO₃ easily because the particles size decreased with decreasing R_w (=[H₂O]/[surfactant]) value as well as nitrate concentration. The electrode prepared by the resulting particles showed high oxygen reduction activity as compared with that prepared by mechanical mixing-method. Furthermore, it was found that new RM method (ROP-RM) using KMnO₄ as an oxidizer gave higher oxygen reduction activity than the HP-RM method, although particle size of LaMnO₃ obtained by the ROP-RM method was almost same as that by RM-HP method.     

Effect of the preparation conditions of carbon-supported Pt catalyst on PEMFC performance

Carbon-supported Pt catalysts were prepared using NaBH₄ as a reducing agent in either ethylene glycol or water for use as a cathode catalyst in PEMFCs (polymer electrolyte membrane fuel cells). Aqueous NaBH₄ solution was used to reduce Pt precursor and to produce the Pt-W catalyst, while Pt-E and Pt-E-base catalysts were synthesized using NaBH₄ in ethylene glycol for the reduction of Pt. Compared to Pt-W catalyst, Pt-E and Pt-E-base catalysts have higher Pt dispersion and larger EAS (electrochemically active surface area) due to the stabilizing effect of ethylene glycolic NaBH₄ solution on Pt particles. In addition, increasing pH of the preparation solution improved the Pt dispersion (Pt-E-base). In unit cell tests the performance of Pt catalysts decreased in the following order: Pt-E-base > Pt-E > Pt-commercial > Pt-W. Higher metal dispersion and larger EAS are believed to be responsible for the superior performance of Pt-E catalysts, particularly Pt-E-base, compared to other catalysts.     

The influence of electrochemical annealing in CO saturated solution on the catalytic activity of Pt nanoparticles

Combining non-destructive, identical location—transmission electron microscopy (IL-TEM) with rotating disk electrode (RDE) measurements, the influence of different treatment procedures on the catalytic activity of carbon supported Pt nanoparticles is probed. IL-TEM shows that the treatment of the catalyst has only minor influence on its structure or the particle shape and size; in particular no treatment induced particle agglomeration is observed. At the same time, both CO stripping and CO bulk measurements are significantly influenced by the electrochemical treatment. In consistence with previous studies this can be explained by the removal of defects in the CO adlayer structure while cycling in CO saturated solution. In contrast, however, it is demonstrated that CO annealing has no impact on the oxygen reduction reaction in the mixed kinetic-diffusion control potential region.     

The effect of pH on oxygen and hydrogen peroxide reduction on polycrystalline Pt electrode

Oxygen reduction (ORR) and hydrogen peroxide reduction (HPRR) reactions were studied on polycrystalline Pt by the rotating disc electrode technique in sulphate solutions over the entire pH range. Initial potentials for both ORR and HPRR coincide with the potential region of PtOH formation and shift negatively with the increase of the pH of the solution. For pHs lower than 3.0 and higher than 10.0, the ORR takes place through 4e-series pathways from acid and alkaline solutions, respectively. For 3.0 < pH < 6.0, the overall number of electrons exchanged depends on the potential and falls below 4 for ORR and below two for HPRR. This indicates that both reactions occur in a limited extent due to the changes of the local pH in the course of these reactions which gives rise to the double wave in the polarization curves (as observed for ORR for pH 3.5 and pH 4.0 and for HPRR for pH 4.0). The change of the Tafel slopes with potential indicate the change in reaction pathway from one that takes place in acid – to one that takes place in alkaline solution.     

Electrocatalytic Activity and Stability of Pt clusters on State-of-the-Art Supports: A Review

Instability of supported Pt clusters due to limited bonding with conventional carbon supports and carbon dissolution leads to significant cathode performance losses with time, impeding the development of commercial proton exchange membrane fuel cells. One approach that has recently been gaining momentum is the use of the electrocatalyst support to enhance both the stability and activity of Pt clusters for the oxygen reduction reaction. This review article focuses on four support types: advanced carbons, conductive ceramics, metallic underlayers for Pt monolayer catalysts, and the 3M crystalline organic whiskers. Advantages and disadvantages of each support are summarized and promising future directions for research in this area are discussed.     

Electroreduction of oxygen on Pt nanoparticle/carbon nanotube nanocomposites in acid and alkaline solutions

The kinetics of O₂ reduction on novel electrocatalyst materials deposited on carbon substrates were studied in 0.5 M H₂SO₄ and in 0.1 M NaOH solutions using the rotating disk electrode (RDE) technique. Pt nanoparticles (PtNP) supported on single-walled (PtNP/SWCNT) and multi-walled carbon nanotubes (PtNP/MWCNT) were prepared using two different synthetic routes. Before use, the CNTs were cleaned to minimize the presence of metal impurities coming from the catalyst used in the synthesis of this material, which can interfere in the electrochemical response of the supported Pt nanoparticles. The composite catalyst samples were characterised by transmission electron microscopy (TEM) showing a good dispersion of the particles at the surface of the carbon support and an average Pt particle size of 2.4 ± 0.7 nm in the case of Pt/CNTs prepared in the presence of citrate and of 3.8 ± 1.1 nm for Pt/CNTs prepared in microemulsion. The values of specific activity (SA) and other kinetic parameters were determined from the Tafel plots taking into account the real electroactive area of each electrode. The electrodes exhibited a relatively high electrocatalytic activity for the four-electron oxygen reduction reaction to water.     

High performance polymer electrolyte fuel cells with ultra-low Pt loading electrodes prepared by dual ion-beam assisted deposition

Ultra-low pure Pt-based electrodes (0.04-0.12 mg_Pt/cm²) were prepared by dual ion-beam assisted deposition (dual IBAD) method on the surface of a non-catalyzed gas diffusion layer (GDL) substrate. Film thicknesses ranged between 250 and 750 Å, these are compared with a control, a conventional Pt/C (1.0 mg_Pt(MEA)/cm², E-TEK). The IBAD electrode constituted a significantly different morphology, where low density Pt deposits (largely amorphous) were formed with varying depths of penetration into the gas diffusion layer, exhibiting a gradual change towards increasing crystalline character (from 250 to 750 Å). Mass specific power density of 0.297 g_Pt/kW is reported with 250 Å IBAD deposit (0.04 mg_Pt/cm² for a total MEA loading of 0.08 mg_Pt/cm²) at 0.65 V. This is contrasted with the commercial MEA with a loading of 1 mg_Pt(MEA)/cm² where mass specific power density obtained was 1.18 g_Pt/kW (at 0.65 V), a value typical of current state of the art commercial electrodes containing Pt/C. The principal shortcoming in this effort is the area specific power density which was in the range of 0.27-0.43 W/cm² (for 250-750 Å IBAD) at 0.65 V, hence much below the automotive target value of 0.8-0.9 W/cm² (at 0.65 V). An attempt to mitigate these losses is reported with the use of patterning. In this context a series of patterns ranging from 45 to 80% Pt coverage were used in conjunction with a hexagonal hole geometry. Up to 30% lowering of mass transport losses were realized.     

The effect of heat treatment on nanoparticle size and ORR activity for carbon-supported Pd-Co alloy electrocatalysts

Carbon-supported Pd-Co alloy electrocatalysts were synthesized and characterized for the purpose of the fuel cell cathode oxygen reduction reaction (ORR). An impregnation method was employed for the synthesis, in which sodium borohydride was used as a reducing agent. The synthesized catalysts were characterized in terms of structural morphology and catalytic activity by XRD, XPS and electrochemical measurements. Surface cyclic voltammetry was used to confirm the formation of the Pd-Co alloy. In order to improve activity and stability, the catalysts were heat-treated in the temperature range of 300 °C to 700 °C. The optimal heat-treatment temperature was found to be 300 °C, where the average particle size of 8.9 nm, and the highest ORR catalytic activity, were obtained. The catalyzed ORR kinetics were also studied using the rotating disk electrode (RDE) method. The kinetic parameters were then obtained. Electrocatalytic ORR activity was also examined in an acidic solution containing methanol. The results showed that the synthesized Pd-Co/C catalyst has methanol tolerant capabilities.     

Recent advances in platinum monolayer electrocatalysts for oxygen reduction reaction: Scale-up synthesis, structure and activity of Pt shells on Pd cores

We have established a scale-up synthesis method to produce gram-quantities of Pt monolayer electrocatalysts. The core-shell structure of the Pt/Pd/C electrocatalyst has been verified using the HAADF-STEM Z-contrast images, STEM/EELS, and STEM/EDS line profile analysis. The atomic structure of this electrocatalyst and formation of a Pt monolayer on Pd nanoparticle surfaces were examined using in situ EXAFS. The Pt mass activity of the Pt/Pd/C electrocatalyst for ORR is considerably higher than that of commercial Pt/C electrocatalysts. The results with Pt monolayer electrocatalysts may significantly impact science of electrocatalysis and fuel-cell technology, as they have demonstrated an exceptionally effective way of using Pt that can resolve problems of other approaches, including electrocatalysts’ inadequate activity and high Pt content.     

On the effect of particle size on heat transfer in vertical upflow of gas-solids suspension

The occurrence of a minimum in heat transfer coefficient with particle diameter in a vertical upflow transport column as reported by Jepson et al. (1963) is explained by the minimum in total surface area of solid particles per unit volume of column with particle diameter. The minimum in the external surface area of solids in the column is shown to be due to the counteracting effects of increasing solids holdup with particle diameter and decreasing specific surface area of solid particles with increasing particle diameter.     

微细物料颗粒大小的测定(续)

4.2 显微镜法 显微镜法包括使用光学显微镜和电子显微镜,是唯一可以观察和测量颗粒的方法。几种显微镜在测定颗粒大小时的性能如表3。     

阴、阳极加湿程度对PEMFC内部传质的影响

为了研究常规流场下阴、阳极增湿程度对电池内部水分布、传递、膜性能及水拖曳系数等的影响,对PEMFC进行二维建模,应用控制容积法对控制方程进行离散,然后求解,得到了电池内部水和反应气浓度、速度分布、膜中电流密度、电势分布及膜中水分布,考察了气体不同增湿程度对质子交换膜电导率及电池内部传质的影响.结果表明,PEMFC中水综合拖曳系数随着阳极加湿程度的增加而增大,随阴极增湿程度的增加而减小,但阳极增湿对水综合拖曳系数的影响比同增湿程度下阴极增湿对水综合拖曳系数的影响大得多.同时,随着阳极加湿程度的升高,质子交换膜（PEM）电导率急剧升高,而阴极加湿程度对PEM电导率的影响只是停留在较小的电流范围之内.故PEMFC在小电流密度工作时,应该使阳极气体充分增湿;而在大电流密度工作时,应该适当降低阳极的增湿程度以降低阴极两相流的机会,从而改善阴极的传质状况.     

Influence of chloride ions on the stability of PtNi alloys for PEMFC cathode

The dependence of the rate of Ni dissolution from PtNi alloys on the chloride concentration was studied electrochemically in 0.5 M HClO₄ at room temperature. Electrodeposited PtNi catalysts were subjected to extensive potential cycling between 20 mV and 1.3 V at various Cl⁻ concentrations and the cyclic voltammograms (CV) response and the oxygen reduction reaction (ORR) activity of the catalysts were determined at different intervals. Energy dispersive X-ray spectroscopy (EDS) and inductively coupled plasma spectroscopy (ICP) analyses were carried out to determine the elemental composition of the alloys and the amount of dissolved Ni at different stages of the potential cycling. It was found that the presence of Cl⁻ increases the rate of Ni dissolution and by this accelerates the dealloying process relative to potential cycling in chlorine-free solutions. Dealloying is most pronounced during the initial stages of potential cycling. Already a small amount of Cl⁻ is sufficient to dissolve the majority of the non-noble metal from the alloys. Even then, under oxygen reduction conditions, the blockage of Pt surface by Cl⁻ is less pronounced for the alloys than for pure Pt catalysts, leading to marginally improved ORR activity for the PtNi alloys at low Cl⁻ concentrations. From a practical point of view, the effect of chloride ion leakage from a commercially available saturated KCl reference electrode on the electrocatalytic activity was also investigated.     

Enhancement of electrocatalytic O<Subscript>2</Subscript> reduction on carbon nanotube-supported Pt alloys nanoparticles in gas diffusion electrodes

The use of Pt binary and ternary alloys prepared by alloying of Pt with transition metals, as catalysts for fabricating of gas diffusion electrodes (GDEs) is reported. Electrocatalytic properties of oxygen reduction reaction (ORR) were evaluated by cyclic voltammetry, electrochemical impedance spectroscopy, polarization experiments and chronoamperometry. The morphology of the GDEs and elemental compositions of the Pt alloys were characterized by X-ray diffraction (XRD) analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The results indicate that the introduction of Pd and Cd as transition metals in Pt alloys provides fast ORR kinetics. The performance of GDEs with Pt–Pd alloy surfaces for ORR was also studied as a function of overall composition and surface atomic distribution of Pt alloys. The results also show that alloying of Pt with transition metals and various amounts of Pt and Pd in the binary catalysts has a large effect on the performance of GDEs for ORR.     

水泥熟料粒径与性能关系的探讨

文章探讨了水泥熟料粒径与其性能的关系,测定了不同粒径熟料的净浆强度,通过X射线荧光分析仪对不同粒径熟料化学成分进行表征,X射线衍射对不同粒径熟料矿物含量和晶型进行表征,并对不同粒径熟料颗粒进行了岩相分析。从实验数据可以看出,熟料粒径越小,其28 d强度越高。粒径小的熟料碱含量、SO3含量更少,CaO含量更多;同时粒径小的熟料的阿利特,特别是M1型阿利特的含量更高。如果能通过调整,控制熟料粒径在合适的范围,对其使用性能的提升会有很大帮助。     

Investigation of the oxygen exchange mechanism on Pt|yttria stabilized zirconia at intermediate temperatures: Surface path versus bulk path

The oxygen exchange kinetics of platinum on yttria-stabilized zirconia (YSZ) was investigated by means of geometrically well-defined Pt microelectrodes. By variation of electrode size and temperature it was possible to separate two temperature regimes with different geometry dependencies of the polarization resistance. At higher temperatures (550–700 °C) an elementary step located close to the three phase boundary (TPB) with an activation energy of ∼1.6 eV was identified as rate limiting. At lower temperatures (300–400 °C) the rate limiting elementary step is related to the electrode area and exhibited a very low activation energy in the order of 0.2 eV. From these observations two parallel pathways for electrochemical oxygen exchange are concluded.The nature of these two elementary steps is discussed in terms of equivalent circuits. Two combinations of parallel rate limiting reaction steps are found to explain the observed geometry dependencies: (i) Diffusion through an impurity phase at the TPB in parallel to diffusion of oxygen through platinum – most likely along Pt grain boundaries – as area-related process. (ii) Co-limitation of oxygen diffusion along the Pt|YSZ interface and charge transfer at the interface with a short decay length of the corresponding transmission line (as TPB-related process) in parallel to oxygen diffusion through platinum.